1. Field of the Invention
This invention relates to a method for processing aqueous sugared juice containing a mixture of at least one aldehyde-function saccharide (aldose) and at least one ketonic-function saccharide (ketose). It is intended to separate and select the ketonic-function saccharides and to eliminate the aldehyde-function saccharides in the form of useable by-products. The invention is especially applicable to aqueous fructose and glucose mixtures with a view to obtaining glucose-free fructose syrups.
2. Description of Related Art
It is common knowledge that in the sugar and starch industries, as in the canning industries, the pressing juice from sacchariferous plants or the effluents from the various transformations of the basic products result directly or indirectly in mixtures of aldehydic saccharides and ketonic saccharides, mainly glucose and fructose.
When sufficiently concentrated, these mixtures are used either for foodstuffs or, recently, in certain industrial sectors (precision chemistry).
In these mixtures, the glucose has an annoying effect, both industrially, as it conditions parasitic reactions in the subsequent processes, and in the area of foodstuffs, in which it considerably increases the calory-content of the fructose syrups manufactured from the sugared juices.
Of the two main methods for separating the glucose from fructose syrups, one uses chromatographic techniques. The major drawback of these processes is in their very high operating cost. Moreover, the fractional crystallization techniques only apply to very fructose-rich syrups that need to be purified.
The present invention provides a new method for separating aldehyde-function saccharides from ketone-function saccharides.
A primary object of this invention is to provide a moderately priced method applicable both to concentrated juices and more diluted juices (from sugar works, starch works, canning effluents etc.) in order to obtain ketonic saccharide solutions of edible quality.
Another object is to enable the valorization of the aldehydic saccharides to be eliminated by transforming them into a product useable in various industrial fields, especially as a base product for manufacturing polymers.
A further object is to obtain very pure ketonic saccharide syrups, especially fructose.
To this end, the method according to the present invention for processing an aqueous sugar juice containing a mixture of at least one aldehyde-function saccharide and at least one ketonic-function saccharide for separating and selecting the ketonic-function saccharides, comprises in a combination:
(a) producing a reactive hydro-organic medium by mixing the sugared juice with a reagent comprising at least one active methylene ketonic compound and a calcium-based catalytic reagent (referred to below as the catalyst),
(b) mixing the medium for enhancing contact between the phases enabling the condensation of the ketonic compound and the aldehyde-function saccharides with the formation of furanic polyalcohol,
(c) allowing the furanic polyalcohol to precipitate in the reactive medium and separating it by physical means from the liquid medium containing the ketonic-function saccharides.
It was observed that the formation of condensation was greatly improved when the hydro-organic medium contained ethanol or methanol, which would, in a preferred embodiment, be added to the medium in the course of operation (a). So, no degradation product pollutes the medium after the reaction.
Thus, the work of the present inventors revealed a condensation reaction which, unexpectedly, had the remarkable property of being totally selective with respect to aldehydic saccharides: in a hydro-organic medium (in a preferred embodiment, water with ethanol or methanol) and in the presence of a calcium-based catalyst, this reaction subjects the aldehydic saccharides, and, in particular, the glucose, to a stoichiometric transformation, and maintains in their entirety the ketonic saccharides and especially the fructose. The aldehydic saccharides are transformed into furanic polyalcohols without a secondary reaction: this body precipitates spontaneously in the hydro-organic reactive medium and so may be separated off, in particular by simple filtration, to produce a liquid phase containing only the ketonicfunction saccharides.
According to a preferred embodiment, the hydro-organic medium is produced by adjusting the quantity of alcohol with respect to the quantity of water, so that the relative proportional content of alcohol in the water is approximately between 80 and 95%. Moreover, in a preferred embodiment, the solvent used is ethanol. These conditions ensure a very high separating efficiency (generally greater than 95%), and enable a ketonic saccharide solution of edible quality to be obtained.
In a preferred embodiment, the active methylene ketonic compound is mixed slightly in excess of the stoichiometric proportions with respect to the aldehyde-function saccharides contained in the sugar juice in order to obtain a total transformation of these aldehydic saccharides into furanic polyalcohol. Furthermore, the calcium-based catalyst, in particular the calcium chloride, may be mixed in molar proportions of between 1 and 6 with respect to the aldehyde-function saccharides contained in the sugared juice.
The ketonic reagent comprises in particular ethyl or methyl acetylacetate or acetylacetone, which have the advantage of being of a non-toxic character in the event of residual traces.
In order to reduce the duration of the reaction, the reactive medium should be heated to a temperature between 40.degree. C. and the reflux temperature of the reactive medium for a duration of between about four to eight hours, In practice, a temperature in the immediate vicinity of the solvent reflux temperature, in particular 78.degree. C. in the case of ethanol (reflux of ethanol/water azeotrope) could be adopted.
The precipitation of the polyalcohol, in a preferred embodiment, takes place cold and may be enhanced by adding water to the medium after the condensation reaction.
The advantage of the method of separation in compliance with the present invention, which produces totally aldehydic saccharide-free ketonic saccharide syrups, can be easily appreciated. These syrups lend themselves to an industrial or food stuff application without parasitic reactions generating toxic products and without the penalizing calory content caused by the presence of glucose.
It is to be noted that laboratory studies have already been carried out as to the condensation reaction of pure glucose (or other pure aldehydic sugars) with the aim of synthesizing furanic polyalcohols. Reference may be made to the following publications which mention this reaction: F. J. LOPEZ APARICIO et al, Carbohydrate Research, vol. 107, 1982, pages 292-295; F. GARCIA-GONZALEZ, Adv. Carbohydr. Chim. 1956, 11, p. 111; J. S. BALLESTEROS, D. J. MCPHEE, D. HERNANDEZ, Bull. Soc. Chim. FR 1982, p 176; M. OKADA, Y. KASHIWABARA, T. IMAMURA, M. YAMADA, M. KAKEHI; Chemical 35 Abstracts, 1967, 67, n.degree., 108947 M.
Nevertheless, these studies were aimed at solely a condensation reaction of pure samples in a synthetic medium and not a separation of one variety in a mixture, and describe or suggest no selective property as to this reaction. On the contrary, these publications, and notably the first mentioned, show that under the operating conditions and given the catalysts used, the condensation reaction affects indiscriminately both glucose and fructose. They thus provide no information directly applicable to solving the problem of separation, subject of the present invention. Furthermore, these studies recommend particular application conditions or specific type of catalyst (Lewis acids, notably ZnCl.sub.2) which are difficult to exploit industrially due to the serious problems which result: random reproductibility, metal contamination during the aqueous phases, difficulties collecting the furanic polyalcohols which crystallize poorly, maximum output (approx. 60%).